You might just wanna pull the heads on those motors.  It will certainly be
more reliable (not to mention much less work)  to inspect the heads then
hope you see any leaks while on the stand.  Mine didn't leak until the
engine was under load.  If I went to all the trouble of pulling the old
engine, I'd wanna drop something in that is known good.  I'd even re-ring it
while on the stand.  But, if you're just worried about the heads, gasket
replacement aint that bad of a job, and the only special tool you need is a
torque wrench.  Gaskets for both sides are around $100.  Take the heads off
those free motors, then pick the best of the three engines, JB-weld if
necessary, glue it back together and you're done.

Brian